Manually-actuable electrical switches are used in many applications and allow a user to connect and disconnect current in an electrical circuit. Such switches can be found in household lighting, flashlights, and battery-operated toys. A simple switch typically has a moveable contact which completes an electrical circuit. In situations requiring a higher degree of safety and/or reliability, two moveable contacts are used, both of which must be connected for the circuit to be made. A typical "two-contact" type of switch is operated by moving a conductive bar against a pair of contacts to provide an electrical path. The bar is moved away from the contacts when the switch is turned off. For some kinds of loads--high current and inductive--a considerable amount of electrical arcing can occur when the bar closes against and releases from the contacts. The heat generated by this arcing can sometimes melt a portion of the metal contact surface, which can result in the bar being "welded" to one of the contacts.
The bar only welds to one contact at a time because arcing only occurs on the side of the bar making or breaking the current path. More specifically, when the bar touches the first contact no current flows. Only when the second contact is touched does current begin to flow. This initiation of current is what can lead to arcing, and potentially, welding of the contacts. Likewise, when the contacts are broken, arcing only occurs at the first of the two contacts to break. Thus, only one contact at a time is subject to becoming welded, and it is extremely unlikely that the switch will become fused in the "on" state in a single contact cycle.
Once the first contact is welded in a two-contact switch, the switch can normally still be operated by making or breaking the connection between the bar and the non-welded contact. However, in this situation arcing is likely to occur between the bar and the non-welded contact in every subsequent contact cycle. Although the user may have no indication that one of the contacts is welded, there is now a substantial risk that both contacts will become welded and the switch will not turn off. In some applications such as motorized toy vehicles, it can be very dangerous for a switch not to turn off.
Previous switch designs used complex mechanisms to prevent the switch from permanently fusing closed. Other simpler designs were not dependable when the switch was required to be frequently operated.
It is therefore an object of the present invention to provide a fail-safe electrical switch that cannot be reactivated when an internal contact becomes welded.
It is a further object of the present invention to provide a fail-safe "two-contact" switch wherein the switch is rendered inoperative when one of the two contacts becomes welded.
It is a further object of the present invention to provide a normally-open fail-safe switch that can be used in applications requiring a high degree of safety.
It is a further object of the present invention to provide a fail-safe electrical switch that is economical to produce.
It is a further object of the present invention to provide a fail-safe electrical switch having a simple structure.